The invention relates to a method for producing fine threads from solutions of polymers of natural or synthetic origin and devices for the production thereof.
Fine threads, also termed microthreads, mainly however microfibres of finite length, have been produced for many years according to a hot air blown spinning method, the so-called meltblown method, and there are various devices for this purpose nowadays. They all have in common that, next to a row of melt borings—also a plurality of rows which are parallel to each other have become known—hot air exits which draws the threads. By mixing with the colder ambient air, the result is cooling and solidifying of these threads or fibres of finite length because often, generally in fact without being desired, the threads tears The disadvantage of these meltblown methods is the high energy outlay for heating the hot air flowing at high speed, a limited throughput through the individual spinning borings (even when these were set increasingly more densely in the course of time, up to a spacing of below 0.6 mm in the case of 0.25 mm in the hole diameter), that the result is tears in the case of thread diameters below 3 μm, which leads to beads and protruding fibres in the subsequent textile composite, and that the polymers are thermally damaged by the high air temperature, significantly above the melt temperature, which is required for producing fine threads. The spinning nozzles, a large number of which has been proposed and also protected, are complex injection tools which must be manufactured to a high precision. They are expensive, subject to faults in operation and complex to clean.
Meltblown methods of this type have also become known for the formation of fibres of finite length made of lyocell materials, i.e. spun from a solvent, generally NMMO (N-methylmorpholine-N-oxide), of dissolved cellulose, e.g. WO 98/26122, WO98/07911, WO99/47733.
In the French patent specification 2 735 794, a method is described in which a cellulose material from one or more spinning borings is split into individual particles by bursting (éclatement) and these are drawn by the gas flow into fibres of finite length. The process of fibre formation takes place in turbulent flow conditions.
A predominant problem when spinning lyocell threads from solution materials is the spinning reliability. Undissolved particles or materials enriched unequally with cellulose lead to thread tears, as a result of which particular care must be taken to avoid these two determining parameters. This leads to special embodiments of the devices, demands on the ambient conditions and a spinning method which must be implemented within narrow limits and hence is sensitive.
The object underlying the present invention is therefore to produce improved methods and devices for producing fine threads from solutions of polymers, which threads are substantially endless, are not thermally damaged by the gas flows drawing them, require a low energy outlay and can be produced by a spinning tool which is simple in its construction.
In the German patent DE 199 29 709 C2, a method and devices is described, according to which substantially endless threads are produced from polymer melts. The molten-liquid polymer threads exit from spinning borings which are disposed in one or more parallel rows or rings and enter into a chamber of a specific pressure, which is filled with gas, generally with air, and is separated from the surroundings, and they proceed into an area of rapid acceleration of this gas at the outlet from the chamber, said outlet being configured as a Laval nozzle.
The forces transmitted on the way there to the respective thread by shear forces increase, its diameter is reduced greatly and the pressure in its still liquid interior increases correspondingly greatly in inverse proportion to its radius due to the effect of surface tension. Due to the acceleration of the gas, its pressure drops in accordance with the laws of fluids. The conditions of the temperature of the spinning material, of the gas flow and its rapid acceleration are thereby coordinated to each other such that the thread reaches a hydrostatic pressure in its interior before its rigidification which is greater than the ambient gas pressure so that the thread splits and divides into a multiplicity of fine threads next to each other. Threads and air leave the chamber by means of a gap at the bottom in said chamber. The splitting takes place in or after the gap and in otherwise unchanged conditions in a surprisingly stable stationary manner at a specific point. In the region of high acceleration, gas- and thread-flow extend in a parallel fashion, the flow boundary layer around the threads being laminar. A continued fanning-out of the original thread monofilament without the formation of beads and tears is achieved. A multifilament of very much finer threads is produced from one monofilament using a gas flow of ambient temperature or a temperature situated somewhat above that.
The threads can be drawn further after the fanning-out point until they are rigid. This occurs very rapidly because of the suddenly produced larger thread surface. The threads are endless. The result can be threads of finite length to a minor degree due to technical interference influences, but the endless fine monofilaments are far more predominant.
The spinning materials used in DE 199 29 709 are meltable polymers. These are available of a synthetic or natural origin. Amongst the fibres based on natural raw materials, in particular those of the secondary growing raw material cellulose are of interest.
It has been shown that these methods of splitting threads can be applied also to lyocell spinning materials, in that cellulose is dissolved in N-methylmorpholine-N-oxide and water and pressed out through spinning borings into threads. Other solvents can also be used, NMMO having proved however to date to be the most suitable. The spinning material present as a solution, as described above, is spun out and the threads run through the air gap prescribed by the Laval nozzle, in which air gap they are drawn into thinner diameters, and proceed subsequently into a water bath in which the cellulose coagulates into the thread and the solvent passes into the water bath which is renewed because of the constant enrichment and the solvent is recycled.
A characteristic feature of the method according to the invention is that the accompanying gas flow, generally air, accompany the liquid solution material threads shortly after their exit from the spinning boring and draw them by transverse stress. As a result, they obtain an orientation and a cooling which both lead to increasing strength and a reduction in the very damaging tears, even as far as their complete prevention. Due to the mixing of the gas flow with the ambient atmosphere, generally also air, the gas flow is in fact slowed down and the threads are no longer subjected to the initial stress due to the higher speed of the same, but remain endless and continue to be carried by the air flow even in the case of tears. The threads are still of the initial soluble material if the precipitation of the cellulose has not already been commenced by injection of e.g. steam or water, These threads can be laid out on a travelling screen and be separated from the accompanying gas flow, as is known in methods for spinning non-woven fabrics, the gas (air) passing through the travelling screen and being suctioned off underneath the same and the threads, which have been laid out into a non-woven material, being only now supplied to the precipitation bath. An otherwise very precise implementation when spinning lyocell threads, beginning with fine capillary diameters for the spinning boring, subsequent air gap and its temperature and renewal and also the requirements for uniformity of the melt to be as free as possible of undissolved parts, which are permitted only in a few ppm, is dispensed with by the compulsory guidance of the threads by the drawing air flow. The thread forming and laying-out space is easily accessible because spacings of in fact 1 and 2 m can be produced between nozzle outlet and collecting strip.
Instead of laying out the threads from the solution material into a nonwoven fabric and subsequently bringing them into a precipitation bath, threads can be spun in the same way according to the method according to the invention and be separated from the accompanying gas flow, in that said gas flow is suctioned off laterally in the device, as provided similarly in the German patent 42 36 514. The individual threads or even a plurality as yarns are then supplied to precipitation devices for coagulation of the cellulose and are wound up on coils.
In contrast to the production of microthreads from synthetic polymers, such as polyethylene, polypropylene, polyamide, polyester and others, the fanning-out of the solution material jet to produce fine and microfine threads is only partly required. As noted before, after removal of the solvent by coagulation there are produced, corresponding to the cellulose content used in the solution material of already a good 10%, i.e. at the concentration which is entirely normal in the case of spinning methods for lyocell threads, threads in the region of below 10 μm in diameter without fanning-out, and it was shown that, only to a minor degree, also because of the particular viscosity behaviour of the NMMO-cellulose solutions which are very different from synthetic polymers, splitting into a plurality of threads next to each other is possible only to a minor degree and in the case of lower cellulose contents of the spinning material. Whilst a temperature increase is adequate in the case of synthetic polymers in order that, because of the effect of surface tension due to the increase in the internal pressure in the thread, the latter bursts and is fanned out into individual threads, damage to these sensitive materials at temperatures significantly above 100° C. results rapidly in the case of lyocell and subsequently the threads lack strength and other desired properties.
In contrast, it has been shown that other natural polymers can be processed into substantially endless threads corresponding to the method according to DE 199 29 709 and the one present here. They behave like synthetic polymers with respect to fanning-out or more like the cellulose materials for lyocell threads according to type.
Another polymer on a natural basis which can be spun into threads is polylactide PLA (polylactic acid), which is obtained on the basis of starch, e.g. cereal or maize starch, but also from whey or sugar. Materials made of PLA have the particular property that they are biodegradable, the degradation, i.e. the decomposition into CO2 and H2O being able to be adjusted also for a specific temporal duration, and that they are body-friendly. Here too, it is achievable with the split spinning method to produce very fine threads, as can be obtained otherwise only with the disadvantages of the meltblown method—large quantities of air must be increased to at least the melt temperature—the polymers generally being damaged.
A further objective is the increase in economic efficiency in the production of the threads due to a higher spinning material throughput and lower specific air and hence energy consumption. It has been shown that thread-forming plastic material solutions of natural or synthetic origin of very different types can be formed not only into threads, in that they are pressed out of round or profiled individual openings and subsequently are drawn by gas or air flows, but that split threads can be produced from films in an entirely similar manner as the monofilaments produced from individual openings. In addition, the spinning material is pressed out of a longitudinally extending slot-shaped nozzle, as mentioned above, into a chamber of a specific pressure, separated from the surroundings, to which gas, e.g. air, is supplied, the film passing into an area of rapid acceleration of the gas at the outlet from the chamber into a longitudinal gap. Underneath the acceleration zone, i.e. in the relaxation zone, the film fans out and there are then produced piles of substantially endless threads, although in contrast to those split from monofilaments these are of very different diameter and have knot-shaped thickenings. These are produced in the still molten-liquid state of the spinning materials and, within certain limits, can be adjusted by the main method parameters of melt temperature, melt throughput and drawing gases—generally air flows—within certain limits. Monofilaments which can then also be wound up cannot thus be produced by splitting films but in fact non-woven fabrics can. These spun non-woven fabrics made of irregularly laid-out monofilaments of different thread diameters can have advantages and are similar rather to natural materials in which a larger spectrum of different individual elements composing them, here for instance fibres and threads, thus occurs as in the case of leather and wood, different monofilaments of which produce their particular and generally advantageous properties.
In both processes, fanning-out of a monofilament or of a film, the temperature of the spinning material exerts the greatest influence because it determines viscosity and hence thread forming capacity and surface tension and hence pressure formation in the monofilament and in the film. Cooling of the thread too prematurely is therefore not desired, in contrast an increase in the temperature shortly before exiting from the spinning opening can be of advantage. The mechanism of fanning-out is similar in the case of the monofilament and the film but is not the same. In the case of monofilaments, the result is splitting when the pressure in the interior is greater than that in the surrounding gas flow. This occurs during the split spinning method as a result of the fact that the thread diameter reduces due to an accompanying gas flow in addition to the generally small influence of gravity, said gas flow constantly accelerating and the pressure in the gas reducing according to the laws of fluids. Due to the surface tension, the pressure in the liquid monofilament becomes greater. In monofilaments, the result is fanning-out due to bursting of the monofilament when the liquid skin can no longer hold the thread together. During spinning out of films, different pressures are produced across the film width and in fact they are higher at the edges due to the surface tension because of the curvature there. Such films are fundamentally unstable even if the gas flow is maintained according to the invention in a laminar fashion for as long as possible. The result is furrows, striations across the film width and ruptures with the formation of thread-shaped or strip-shaped individual parts, also termed ligaments.
The area of high acceleration and pressure drop in the gas flow is produced according to the invention in the form of a rotation-symmetrical or longitudinally extending Laval nozzle with a convergent contour towards a narrowest cross-section and then rapid widening, the latter in fact in order that the newly formed monofilaments which run beside each other cannot adhere to the walls. In the narrowest cross-section, in the case of corresponding choice of the pressure in the chamber (in the case of air, approximately twice as high as the ambient pressure behind it), the speed of sound can prevail, and in the widened part of the Laval nozzle, supersonic speed.
For the production of thread non-woven fabrics (spun non-woven fabrics), spinning nozzles with spinning borings disposed in lines and in a rectangular form or with a slot form and Laval nozzles with a rectangular cross-section are used. For the production of yarns and for particular types of non-woven fabric production, round nozzles with one or more spinning borings and rotation-symmetrical Laval nozzles can also be used.
The advantage of the present invention resides in the fact that microthreads in the range below 10 μm, for example between 2 and 5 μm, can be produced in a simple and economical manner, which is accomplished in the case of simple drawing for instance by the meltblown method only with hot gas (air) jets which are heated above the melting point and hence requires significantly more energy. In addition, the threads are not damaged in their molecular structure by excess temperatures, which would lead to reduced strength, as a result of which they can then often be rubbed out of a textile web. A further advantage resides in the fact that the threads are endless or quasi endless and do not protrude out of a textile web such as a non-woven fabric and cannot be detached as bits of fluff. The device for executing the method according to the invention is simple. The spinning borings of the spinning nozzle, just as the slot nozzle, can be larger and hence less susceptible to faults. The Laval nozzle cross-section does not require in its precision the narrow tolerances of the lateral air slots of the meltblown method. In the case of a specific polymer, only the solution temperature and the pressure in the chamber require to be coordinated to each other and with a given throughput per spinning boring and the geometrical position of the spinning nozzle relative to the Laval nozzle, the result is fanning-out. In the case of lyocell, the solution thread is thinned to the desired diameter, the fanning-out only occurs sporadically.
It is a development of the invention to cool the solution cone, which is round as a monofilament or cuneiform as a film, as little as possible before the fanning-out and furthermore to heat it to a higher temperature. For this purpose, heating devices, which are screened relative to the gas flow, are fitted on both sides of the outlet openings—row of borings or slot. These heating devices direct heat on the one hand in the region of the outlet opening to the spinning material from the exterior and, where it permits a higher speed and hence higher heat transition, give it a temperature increase, on the other hand the heating devices are of the type that transmit heat by radiation to the cone-shaped or cuneiform part of the spinning material which is being formed.
In FIG. 1, a section through the lower part of a spinning nozzle 1 and an assigned Laval nozzle is illustrated, this section applying both for a rotation-symmetrical spinning nozzle, which spins a thread or a monofilament, and for a rotation-symmetrical Laval nozzle, and for a slot-shaped or rectangular spinning nozzle, which spins a film, and corresponding to a rectangular Laval nozzle. There can also be provided a spinning nozzle with a plurality of spinning borings disposed in a row with corresponding longitudinally extending Laval nozzle. Underneath the spinning nozzle 1 there is located a plate 11, 11′ with a gap 12′ which, observed from the spinning nozzle, has a convergent and then slightly divergent configuration and widens out sharply at the lower edge of the plate 11, 11′, as a result of which the Laval nozzle is formed. The spinning nozzle or the spinning borings of the spinning nozzles terminate shortly above the Laval nozzle or in the upper plane of the plate 11, 11′, if necessary the spinning nozzle 1 can also protrude slightly into the opening 12.
Between the spinning nozzle 1 and the plate 11, 11′ there lies a sealed chamber to which gas is supplied, for example by a compressor, corresponding to the arrows 6, 6′. The gas, which can be air, has normally ambient temperature but can also have a somewhat higher temperature, for example 70° to 80°, because of the compression heat from the compressor. The spinning nozzle 1 is surrounded by an insulation arrangement 8, 8′ which serves for screening the spinning nozzle heated to spinning temperature against heat losses, an air gap 9 being advantageously provided also between the spinning nozzle 1 and the insulation arrangement 8, 8′. The spinning nozzle 1 has an outlet opening 4, in the region of which a heating device 10, 10′ is fitted which in the embodiment is configured as a flat heating strip and which is insulated in an advantageous manner relative to the insulating arrangement 8, 8′ in order to avoid heat losses by parts 13 and 13′. The chamber underneath the plate 11, 11′ normally has ambient pressure, i.e. atmospheric pressure, whilst the gas in the chamber between the spinning nozzle 1 and the plate 11, 11′ is at an increased pressure. In the case of directly subsequent further processing into non-woven fabric, yarns or other thread structures, the chamber underneath the plate 11, 11′ can have a pressure which is somewhat increased relative to atmospheric pressure, for example by a few millibars, which is required for the further processing, such as laying of the non-woven fabric or other thread collecting devices.
A polymer solution 2, i.e. for example lyocell prepared by dissolving cellulose in a solvent, such as amine oxide, flows along the illustrated arrow 3 towards the outlet opening 4 of the nozzle 1. A thread 5 or a film is formed which, in its further course because of the gas flow, which extends along the illustrated arrows 6, 6′, coming laterally from above between the contour of the faces of the plate 11, 11′ and the outer faces 7, 7′ of the insulation arrangement 8, 8′, is reduced in diameter or in width. The heating device 10, 10′ heats the capillary of the outlet opening 4 from the exterior and, by corresponding lengthening, can essentially heat the spinning material flowing past it by radiation with its lower part. The thread 5 or the film passes into the constriction 12′ of the flow cross-section formed by the parts 11, 11′ of the plate for the gas flow 6, 6′ according to the type of Laval nozzle with the narrowest cross-section at 12. Until there, the flow velocity of the gas increases constantly and the speed of sound can prevail in the narrowest cross-section 12 if the critical pressure ratio for instance in the non-operative state of the gas p1 in the chamber above the plate 11, 11′ relative to the pressure in the narrowest place pe is exceeded. Due to the widening of the Laval nozzle towards the chamber with the pressure p2 beneath the plate 11, 11′, even supersonic speeds can be produced with supercritical pressure ratios. In general, the Laval nozzle widens very sharply immediately after the narrowest cross-section 12 or shortly thereafter in order to avoid adhesion of the threads to the plate 11, 11′ due to the fanning-out beginning in this region shortly beneath the Laval nozzle.
In the illustrated example, the thread 5 splits or fans out when the thread casing can no longer hold together the solution thread against the internal pressure which has increased with the thread constriction. The monofilament then divides into individual threads which cool rapidly because of the temperature difference between the solution and the cold gas or air and the suddenly greatly increased surface of the monofilaments, relative to the thread material. Thus a specific number of very fine, substantially endless monofilaments are produced. In the case of a lyocell solution, the phenomenon of fanning-out frequently does not occur or only here and there, i.e. in FIG. 1 the thread which is spinning out would continue. The thread is drawn by the laminar gas flow at a constantly increasing speed so that in conclusion the result is fine threads because of the proportion of cellulose being at or below 10%.
The soluble film also rips shortly beneath the Laval nozzle, the pressure ratios in the film before the fanning-out being different across the width and the film becoming unstable. Shortly before fanning-out, the result is furrows and striations across the width of the film and then ruptures of the threads with small, but larger diameters.
It follows from the nature of splitting processes of this type that the number of threads produced after the fanning-out point, which can still be in the Laval nozzle or for example 5 to 25 mm under the narrowest point of the Laval nozzle, may be non-constant. Because of the short route which thread or film and gas cover together up to the fanning-out point or up to the final drawing of the thread, the flow boundary layer around the thread is laminar. The air from the incoming pipes is also directed in as laminar a fashion as possible to the area of the fanning-out. This has the advantage of smaller flow losses but also of a more uniform temporal course of the fanning-out. The accelerated flow, as occurs in the cross-section of the laval nozzle, remains laminar and can even be laminated if a certain turbulence prevails in advance.
FIG. 2 shows the perspective view of a system for the method according to the invention in which a lyocell material 130 is supplied to a device 30 and a non-woven fabric 20 is obtained therefrom. The device 30 for producing substantially endless threads corresponds to the arrangement according to FIG. 1, a plurality of spinning nozzles or spinning borings being disposed in a row corresponding to FIG. 1 and the Laval nozzle extending longitudinally or respectively having a rectangular configuration. Monofilament threads exit from the individual spinning borings, are tapered by the transverse stresses of the gas flow and fan out if necessary, however less with lyocell, in the lower part of the gap of the non-illustrated Laval nozzle or somewhat thereunder into a plurality of threads. With lyocell, essentially monofilaments are spun.
The airflow accompanying it leads it to a collecting strip 50, where the threads which are still dry are laid down. This is possible in the present method and has great advantages relative to lyocell methods in which the threads are introduced immediately after a short air gap of a few cm into the precipitation bath, generally of water. Underneath the laying-out stretch in the dry place, there is located a suction device, illustrated by the box 60 as is common with spun non-woven fabric methods so that the accompanying air is discharged by non-illustrated suction devices. In order to implement an introduction beneath the level of the precipitation bath 70, without the threads detaching from the travelling screen, a precipitation bath liquid, predominantly water, can also be suctioned through the travelling screen at this point, not shown in detail, or a roller 89 with or without contacting the water surface is present which presses the non-woven fabric into the precipitation bath 70. While the collecting strip 50 is moved back, the non-woven fabric 20 proceeds for further processing thereof, for example by calendering, drying and further processes such as water jet compacting.
The air can be in part discharged already in advance along the arrows 120, 120′, the boxes 110, 110′ thereby have non-illustrated air-permeable faces orientated towards the threads.
Suction devices of this type laterally to the thread bundle can be used in a particular manner if the threads are not intended to be processed into a non-woven fabric but into an endless yarn, which is intended to be wound onto rolls or cut into staple fibres, respectively after solvent and cellulose material have been separated from each other in advance by coagulation.
It is a particular peculiarity of the method according to the invention that, after their exit from the spinning borings and possibly after their splitting, the threads experience shear forces due to the gas flow, generally air flow, extending substantially parallel to them. Hence it differs from the forces otherwise applied for spinning by winding or other types of take-off devices. The spinning solution from the spinning borings withstands only low tensile forces and it is therefore not possible with methods according to the state of the art to produce very fine threads because the spinning material can be drawn to a thread of a small diameter only in the air gap between the nozzle outlet and the coagulation bath, and no longer thereafter. According to the present method, the forces required for the forming are transverse stress forces (in addition to the very low effect of gravity) which do not stress the thread as tensile forces across the thread cross-section, as a result of which tearing scarcely occurs.
The coagulation of the dissolved thread polymer, here cellulose for lyocell threads, in a solvent, here NMMO, can be introduced already between the spinning device 30 and the laying-out surface 51, in that water mist or steam are injected laterally against the thread bundle, i.e. for instance where the previously described suction boxes for air 110, 110′ are fitted and hence, precisely in the reverse manner to the discharged air, now moist air or steam are introduced into the thread bundle. This has the effect that the threads are enriched in the cellulose proportion on their exterior already in front of the system and binding to each other is not as great as when they are laid out to form a non-woven fabric without the same. The non-woven fabric is then introduced into a precipitation bath, the result being self-binding subsequently due merely to pressure rollers or between a drum, also heated, and the travelling screen. Because the produced lyocell threads are soft and adhere already to each other if they are connected to each other at only low pressure. This autogenic connection is a further particular advantage in the production of non-woven fabrics made of lyocell threads. If the coagulation is already introduced, then the binding is not so strong and softer non-woven fabrics with a textile texture are obtained relative to the previously non-sprayed non-woven fabrics drawn only through the precipitation bath which are more compact and have a harder paper texture.
It is understood that yet further steps of coagulation or washing out of the solvent can be added after the trough illustrated in FIG. 2. For this purpose, perforated cylinder washing machines can also be used, as are used in the textile industry, the non-woven fabric looping round the perforated cylinder in a specific circumferential segment and the water being withdrawn axially through the non-woven fabric and the perforated cylinder casing and being supplied once again to the bath or for separation of water and solvent, for example NMMO. Subsequently, the non-woven fabric must be dried, for which purpose perforated cylinder dryers can be used. Since in general a high shrinkage of the lyocell threads occurs here, the non-woven fabric can be guided between a suction cylinder which is subject to a warm air flow and a travelling screen looping round the latter and moving at the same speed.